The universal use of electronic systems in aircraft for a multiplicity of purposes requires that the electronic components are maintained at suitable operating temperatures to assure proper performance and to contribute to long term reliability.
One known approach to cooling airborne electronic components makes use of ram air that the aircraft receives during flight, for example as disclosed in U.S. Pat. No. 2,706,255. However, this known approach does not make optimum use of the cooling capabilities of the ram air. More specifically, the cooling capabilities of ram air during relatively low speed flight are significantly different from the cooling capabilities of ram air during high speed flight. Yet, this known approach does not take advantage of special features of high speed ram air for cooling. Other known uses of ram air involve using the ram air to perform both heating and cooling functions to maintain desired cabin temperatures and pressures. Such uses are disclosed in U.S. Pat. Nos. 2,412,110, 2,473,496, 2,669,101, 2,767,561, and 2,916,890. In none of the known uses of ram air is advantage taken of special capabilities of high speed ram air for cooling electronic components in the aircraft.
More specifically, an air turbine may be used to extract useful work from ram air and thereby precool the ram air prior to cooling the electronic components. The turbine cooling capacity is a function of available ram pressure in accordance with the following formula: EQU P.sub.Ram =P.sub.o [1+.2M.sup.2 ].sup.3.5
where P.sub.Ram is available ram pressure, P.sub.o is ambient pressure, and M is Mach number or aircraft speed.
Thus, at relatively high aircraft speeds, cooling capacity of the ram air is relatively high.
Conversely, at relatively low aircraft speeds, ram pressure for driving an air turbine is reduced along with corresponding ram air flow. At lower speeds, better cooling of electronic components can be achieved if the turbine is bypassed and the electronic components are cooled by ram air directly.
In view of the relationship between aircraft speed and ram air pressure, it would be desirable to provide an apparatus for cooling electronic components in an aircraft that takes advantage of the higher cooling capabilities of ram air during high speed flight.
Furthermore, because aircraft speed can vary over a wide range, it would be desirable to provide an apparatus for cooling electronic components in an aircraft that permits automatic switching from a high speed cooling mode to a low speed cooling mode and vice versa.